This invention relates generally to video tape recording systems and, more particularly, to improvements in methods and apparatus for compensating for time base errors in video signals reproduced in such recording systems.
Standard broadcast format video signals contain time base or horizontal synchronizing pulses occurring at equally-spaced intervals, with video information occurring between the pulses. In color transmission, the video information includes both luminance and chrominance data.
Time base errors in a video signal are variations in the normally precise spacing of the synchronizing pulses. Such errors arise in longitudinal video tape recording systems because the systems generally cannot maintain the speed of the tape sufficiently constant as it is moved past the system's record/playback transducer. These tape speed variations can be due to acoustic vibrations in the tape, variations in the tension of the tape as it moves past the transducer, irregularities in the operation of various tape drive elements and other causes.
Recording systems generally introduce time base variations during both the recording and the reproduction of video signals. The frequency spectrum of the timing variations is generally in the range of 5 Hz. to 3000 Hz. and the amplitude of the timing variations can be as high as .+-.5 microseconds, depending on the physical characteristics of the particular recording system under consideration.
An example of apparatus used in the past to compensate for the above-described time base error phenomenon is disclosed in U.S. Pat. No. 3,959,815, issued in the name of G. Rotter et al and entitled "Arrangements for Time Base Error Compensation." The apparatus described in that patent includes a feedback loop in which the synchronizing pulses of a video signal produced in a recording system are locked onto a fixed reference frequency corresponding to the average rate of the pulses. The loop utilizes a variable delay line in the form of a "bucket brigade" or a charge-coupled device, in which the frequency of a clock signal controlling the device is continuously varied so as to compensate for the detected errors in timing. This provides a delayed video signal in which the successive synchronizing pulses are substantially evenly spaced.
Unfortunately, such prior art systems have not provided entirely satisfactory performance when typical commercially available charge-coupled devices are used. These devices suffer from the drawback that they introduce voltage offsets into the video output signals, with the magnitude of the offsets varying according to the frequency at which the devices are clocked. The frequency spectrum of the voltage offsets thus corresponds generally to the spectrum of the variable rate clock signal applied to the device. These voltage offsets create objectionable horizontal streaking in the resulting video picture, i.e. adjacent groups of horizontal lines have respectively greater and lesser luminance levels that would otherwise occur.
Prior attempts to remove the aforedescribed streaking in the video picture have included the use of conventional clamping circuits following the charge-coupled devices, to clamp the synchronizing pulses to a fixed reference level. To remove the streaking completely, the clamping circuits must be fast enough to detect and respond to the entire frequency spectrum of the voltage offset variations, i.e. several thousand Hertz. Use of such a fast clamping circuit, however, frequently creates an objectionable flicker in the resultant video picture.
This flicker is a result of random noise, i.e., noise other than the voltage offset variations introduced by the charge-coupled device, superimposed on the synchronizing pulses. Such noise frequently causes the clamping circuit to adjust the video signal to the wrong level, resulting in random variations in the luminance level (i.e. flicker) of the resulting video picture. Flicker has been found to occur even where the signal-to-noise ratio of the video signal exceeds 40 db.
It will be appreciated from the foregoing that there is still a real need for an effective technique for the correction of time base errors in a video signal, that does not introduce objectionable streaking or flicker in the resulting video picture. The present invention fulfills this need.